Abstract.
Environmental temperatures that exceed body temperature (Tb) force endothermic animals to rely solely on evaporative cooling to dissipate heat. However, evaporative heat dissipation can be drastically reduced by environmental humidity, imposing a thermoregulatory challenge. The goal of this study was to investigate the effects of humidity on the thermoregulation of desert birds and to compare the sensitivity of cutaneous and respiratory evaporation to reduced vapor density gradients. Rates of evaporative water loss, metabolic rate, and Tb were measured in birds exposed to humidities ranging from ~2 to 30 g H2O m-3 (0%-100% relative humidity at 30C) at air temperatures between 44 and 56C. In sociable weavers, a species that dissipates heat primarily through panting, rates of evaporative water loss were inhibited by as much as 36% by high humidity at 48C, and these birds showed a high degree of hyperthermia. At lower temperatures (40-44C), evaporative water loss was largely unaffected by humidity in this species. In Namaqua doves, which primarily use cutaneous evaporation, increasing humidity reduced rates of evaporative water loss, but overall rates of water loss were lower than those observed in sociable weavers. Our data suggest that cutaneous evaporation is more efficient than panting, requiring less water to maintain Tb at a given temperature, but panting appears less sensitive to humidity over the air temperature range investigated here.

This study was conducted at Leeupan farm, Northern Cape, South Africa, during February 2013.

The Sociable Weaver relies mainly on panting for evaporative cooling, resulting in very high rates of evaporative water loss at high temperatures. This would result in rapid dehydration in the absence of access to free water. This suggests that it would have difficulty surviving during long periods of hot, humid weather, eg during a heat wave in the summer rainy season, especially if drinking water is not available.